This invention relates to multilayer coatings having a clear top coat formed on a color base coat and a method for forming such coatings on a substrate.
The exterior of automobiles, for example, is finished with a color base coat and a clear top coat formed on the color base coat for decorative and protective purposes. For higher productivity, the clear top coat is conventionally applied on the base coat wet-on-wet and cured simultaneously with the base coat. This method is highly suitable for inline coating operations in the automobile industry and gives a high grade finish in terms of appearance, weatherability, anti-solvent and chemical resistances, anti-discoloring resistances and the like.
In order to achieve excellent appearance, particularly excellent metallic glamor, it is imperative that the top coat applied on the base coat wet-on-wet does not cause intermixing of the two layers which, if occurs, greatly impairs the orientation of metallic flakes and the metallic glamor. For this reason, attempts have been made to decrease the compatibility between the base coat and the top coat by, for example, using a resin having a higher molecular weight for the base coat than for that of the top coat or by using different resins for different coats such as the combination of acrylic top coat/polyester or cellulose acetate butylate base coat. The compatibility between uncured two coats may also be decreased by modifying coating conditions thereof. This technique includes two-stage application of the base coat, prolonged rest intervals between application steps, elevation of the viscosity of the base coat relative to the top coat and the like. However, none of these known attempts is completely satisfactory. The use of high molecular weight resins requires a decrease in their contents at the time of application. When different resins are used for different coats, the adhesion between different coats will be decreased. Modification of coating conditions increases the number of steps and the time required for the overall coating operation.
One approach for improving aesthetic properties of a multicoat system is to provide a relatively thick top coat on the base coat. In a two coat system comprising a base coat containing aluminum flakes of 10 to 50 .mu.m size, large aluminum flakes often protrude above the base coat surface. The clear top coat therefore must have a film thickness sufficient to compensate these protrusions. Thick clear top coats are effective also for improving the build up appearance of the entire finish when the base coat is pigmented with non-metallic solid color pigments. However, with conventional top coat compositions, the film thickness is limited to only 20-30 .mu.m with a single coating operation, or 40-45 .mu.m with two coating operations. This is because the conventional coating compositions tend to excessively run with an increase in the amount applied per unit area. Thick top coats may be provided by multiple coating operations. However, this technique is less efficient and requires an extensive modification of existing production lines.
Recently, from the viewpoint of economizing natural resources and energy and because of the requirements for pollution control, much research has been done for ways of increasing the nonvolatile contents of coating materials. High-solids coating systems are generally formulated by lowering the molecular weight of vehicle resins but this technique, when applied to two coat systems to be applied wet-on-wet, presents several serious problems such as poor metallic flake orientation, intermixing, poor gloss, excessive run and the like. Another approach would be to incorporate a non-aqueous resin dispersion into the system. However, experiments have shown that this method suffers from the above-mentioned problems.
It is therefore a principal object of the present invention to provide a multicoat system which may obviate the above-mentioned disadvantages. Other objects and advantages of this invention will become apparent as the description proceeds.